It is known that solid metal nanoparticles (i.e. solid, single metal spheres of uniform composition and nanometer dimensions) possess unique optical properties. In particular, metal nanoparticles (especially the coinage metals) display a pronounced optical resonance. This so-called plasmon resonance is due to the collective coupling of the conduction electrons in the metal sphere to the incident electromagnetic field. This resonance can be dominated by absorption or scattering depending on the radius of the nanoparticle with respect to the wavelength of the incident electromagnetic radiation. Associated with this plasmon resonance is a strong local field enhancement in the interior of the metal nanoparticle. A variety of potentially useful devices can be fabricated to take advantage of these specific optical properties. For example, optical filters or chemical sensors based on surface enhanced Raman scattering (SERS) have been fabricated.
A serious practical limitation to realizing many applications of solid metal nanoparticles is the inability to position the plasmon resonance at technologically important wavelengths. For example, solid gold nanoparticles of 10 nm in diameter have a plasmon resonance centered at 520 nm. This plasmon resonance cannot be controllably shifted by more than approximately 30 nanometers by varying the particle diameter or the specific embedding medium.
Metal colloids have a variety of useful optical properties including a strong optical absorption and an extremely large and fast third-order nonlinear optical (NLO) polarizability. These optical properties are attributed to the phasic response of electrons in the metallic particles to electromagnetic fields. This collective electron excitation is known as plasmon resonance.
At resonance, dilute metal colloid solutions have the largest electronic NLO susceptibility of known substances. However, the utility of these solutions is limited because their plasmon resonance is confined to relatively narrow wavelength ranges and cannot readily be shifted. For example, silver particles 10 nm in diameter absorb light maximally at approximately 355 nm, while similar sized gold particles absorb maximally at about 520 nm. These absorbance maximums are insensitive to changes in particle size and various dielectric coatings on the particles.
One method of overcoming this problem is to coat small nonconducting particles with these metals. For example, the reduction of Au on Au2S (reduction of chloroauric acid with sodium sulfide) particles has been shown to red shift the gold colloid absorption maximum from 520 nm to between approximately 600 nm and 900 nm, depending on the amount of gold deposited on the Au2S core and the size of the core. Zhou, et al. (1994). The ratio of the core radius to shell thickness can be controlled by changing the reactant concentrations or by stopping the reaction. In this case, the diameter of the particle core is directly proportional to the red shift in the wavelength of light that induces gold plasmon resonance. However, gold-sulfide particle diameters are limited to sizes of approximately 40-45 nm with a thin gold shell (less than 5 nm). The limited size of the gold-sulfide particles of Zhou et al. limits the absorbance maximum to wavelengths no larger than 900 nm. See, also Averitt et al. (1997) An additional limitation of such particles as defined by Zhou et al. is that both the core and the shell are grown as a result of a single chemical reaction, thus limiting the choice of the core material and the shell material to Au2S and Au respectively. In addition, only the ratio of the core radius to shell thickness may be controlled; independent control of the core radius and the shell thickness is not possible.
Neideljkovic and Patel (1991) disclosed silver-coated silver bromide particles that are produced by intense UV irradiation of a mixture of silver bromide, silver, sodium dodecylsulfate (SDS) and ethylenediamine tetraacetic acid (EDTA). The Neideljkovic particles range in size from approximately 10 to 40 nm and are irregularly-shaped, as determined by transmission electron micrography. Predictably, the spectra obtained from these particle preparations are extremely broad.
In U.S. Pat. No. 5,023,139, Bimboim et al. disclosed theoretical calculations indicating that metal-coated, semi-conducting, nanometer-sized particles containing should exhibit third-order nonlinear optical susceptibility relative to uncoated dielectric nanoparticles (due to local field enhancement). Their static calculations were based on hypothetical compositions. The preferred embodiments disclosed by Birnboim et al. are, in fact, not particles with metallic shells on their surfaces. In those embodiments theoretically proposed by Bimboim et al. that do in fact propose a metal outer shell, there is an additional requirement as to the specific medium in which they must be used in order to properly function.
However, Bimboim does not disclose methods for preparing the disclosed hypothetical compositions. Furthermore, Birnboim's calculations do not take into account surface electron scattering. Surface electron scattering strongly modifies the optical response of all metallic structures that possess at least one dimension smaller than the bulk electron mean free path (e.g. in Au at room temperature the bulk electron mean free path is about 40 nm). This effect reduces the local field enhancement factor that in turn reduces the resonant third order nonlinear optical susceptibility associated with the nanoshell geometry. See, Averitt et al. (1997). Since typical shell thicknesses for these compositions fall below 40 nm, Birnboim et al's theoretical calculations fail to account for this effect which is an important aspect of the optical response for functional metal nanoshells. Finally, it is important to realize that the hypothetical metal nanoshells of Bimboim pertains specifically to the enhancement of the third order nonlinear optical susceptibility.
Moreover, Birnboim-type particles are by definition particles much smaller than a wavelength of light (less than 0.10 times a given wavelength of light), and are particles in which the dielectric property of the nanoshell (in those instances where it is in fact a metal shell that is used in Bimboim et al.) are defined as the bulk dielectric property of the metal selected. In practice, this requires these smaller-than-a-wavelength particles to have metal shell layer thicknesses of many nanometers (e.g., for Au, such minute particles meeting the theoretical requirements of the Birnboim calculations and the bulk dielectric properties of Au required thereby, would necessarily have shells at least 40 nm in thickness). The physical limitations placed on the construction of such particles is therefore considerable.
Methods and materials are needed that can be used to shift the wavelength of maximum absorption of metal colloids. Methods for producing materials having defined wavelength absorbance maxima across the visible and infrared range of the electromagnetic spectrum are needed Particularly, such metal nanoshell composites should be constructed in a manner to allow a choice of core material, core dimensions, and core geometry independent of those criteria for the shell material. Compositions produced by these methods should have relatively homogeneous structures and should not have to rely on suspension in a particular medium in order to exhibit their desired absorption characteristics. Moreover, materials and methods are needed that are not limited in the radial dimensions of the shell layer by the bulk dielectric properties of the metal selected, and are not limited in size to much smaller than a wavelength of light. Materials impregnated with these compositions could be used in such diverse applications as optical switching devices, optical communication systems, infrared detectors, infrared cloaking devices, passive solar radiation collection or deflecting devices and the like.